Nowadays, steel sheets subjected to a surface treatment and thereby provided with a rust prevention property, in particular, galvanized steel sheets or galvannealed steel sheets which are excellent in terms of rust prevention property, are used as material steel sheets in the fields of, for example, automobiles, domestic electric appliances and building material industries. In addition, application of high strength steel sheets to automobiles is promoted to achieve a decrease in the weight and an increase in the strength of automobile bodies by decreasing the thickness of the materials of automobile bodies by increasing the strength of the materials from the viewpoint of an increase in the fuel efficiency of automobiles and the collision safety of automobiles.
In general, a galvanized steel sheet is manufactured by using a thin steel sheet, which is manufactured by hot-rolling and cold-rolling a slab, as a base material, by performing recrystallization annealing on the base material in an annealing furnace of a CGL and by thereafter galvanizing the annealed steel sheet. In addition, a galvannealed steel sheet is manufactured by further performing an alloying treatment on the galvanized steel sheet.
It is effective to add Si and Mn to increase the strength of a steel sheet. However, Si and Mn are oxidized and form oxidized materials of Si and Mn on the outermost surface of the steel sheet even in a reducing atmosphere of N2+H2 in which oxidation of Fe does not occur (oxidized Fe is reduced). Since the oxidized materials of Si and Mn decrease wettability between molten zinc and base steel sheet when a plating treatment is performed, bare spots frequently occur in the case of a steel sheet containing Si and Mn. In addition, even if bare spots do not occur, there is a problem in that coating adhesiveness is poor.
As a method of manufacturing a galvanized steel sheet using a high strength steel sheet containing a large amount of Si as a base material, Japanese Unexamined Patent Application Publication No. 55-122865 discloses a method in which reduction annealing is performed after an oxidized film has been formed on the surface of a steel sheet. However, the effect of JP '865 is not stably achieved. To solve this problem, Japanese Unexamined Patent Application Publication Nos. 4-202630, 4-202631, 4-202632, 4-202633, 4-254531, 4-254532 and 7-34210 disclose methods in which the oxidation rate or reduction amount is specified or in which the oxidation or reduction conditions are controlled on the basis of measurement results of the thickness of an oxidized film in a oxidation zone to stabilize the effect.
In addition, as a galvanized steel sheet which is made from a base material that is a high strength steel sheet containing Si and Mn, Japanese Unexamined Patent Application Publication No. 2006-233333 discloses a method in which the content ratios of oxides containing Si which are present in a coating layer and base steel of a galvannealed steel sheet are specified. In addition, Japanese Unexamined Patent Application Publication No. 2007-211280 specifies, as JP '333 does, the content ratios of oxides containing Si which are present in a coating layer and base steel of a galvanized and galvannealed steel sheet. In addition, Japanese Unexamined Patent Application Publication No. 2008-184642 specifies the amount of Si and Mn which are present in the form of oxides in a coating layer.
To highly increase the strength of a steel, it is effective to add chemical elements such as Si and Mn, which are effective for solid solution strengthening, as described above, and it is possible to increase hardenability of a steel and achieve a good balance of strength and ductility even in the case of high strength steel by further adding Cr. In particular, since press forming has to be performed in the case of a high strength steel sheet which is to be used for automobiles, there is a strong demand for an increase in the balance of strength and ductility.
It was found that, in the case where the methods of manufacturing a galvanized steel sheet which are disclosed by JP '865, JP '630, JP '631, JP '632, JP '633, JP '531, JP '532 and JP '210 are applied to steel in which Cr is added to a steel containing Si, sufficient coating adhesiveness is not necessarily achieved because oxidation in an oxidation zone is suppressed.
In addition, it was also found that, in the case where the methods of manufacturing a galvanized steel sheet which are disclosed by JP '865, JP '630, JP '631, JP '632, JP '633, JP '531, JP '532 and JP '210 are applied to steel in which Mn is added to a steel containing Si, good corrosion resistance is not necessarily achieved because crystal grains in the base steel are taken into a coating layer due to excessive internal oxidation in the case where an alloying treatment is performed.
In addition, it was found that, although good fatigue resistance is achieved using the methods which are disclosed by JP '333, JP 280 and JP '642 in the case of a galvanized steel sheet which is not subjected to an alloying treatment, there are cases where sufficient fatigue resistance is not always achieved in the case of a galvannealed steel sheet which is subjected to an alloying treatment. The methods disclosed by JP '333 and JP '280 are intended to increase coating wettability and phosphating performance, but fatigue resistance is not considered.
It could therefore be helpful to provide a high strength galvanized steel sheet excellent in terms of coating adhesiveness which is made from a base material that is a high strength steel sheet containing Si, Mn, and Cr and a method of manufacturing the galvanized steel sheet.
It could also be helpful to provide a high strength galvanized steel sheet excellent in terms of corrosion resistance and fatigue resistance which has been subjected to an alloying treatment.